In basic chemistry, mixtures are broadly classified into solutions, suspensions, and colloids based on the particle size of the substance distributed within a medium.
Solutions (True Solutions)
A true solution is a homogeneous mixture of two or more substances where the particles are distributed uniformly at the molecular or ionic level. It consists of two components: the solute (the substance that gets dissolved) and the solvent (the medium that does the dissolving).
Key Characteristics of Solutions
- Particle Size: The solute particles are extremely small, with a diameter of less than 1 nanometer (<1 nm or <10−9 m).
- Visibility: Particles are invisible to the naked eye and cannot be seen even under a powerful microscope.
- Homogeneity: The composition and properties are completely uniform throughout the entire volume.
- Stability: Highly stable; the solute particles do not settle down at the bottom of the container when left undisturbed for any duration.
- Filterability: Solute particles pass easily through standard filter paper as well as semi-permeable membranes.
Classification of Solutions Based on Physical State
| Type of Solution | Solute | Solvent | Common Examples |
|---|---|---|---|
| Gas in Gas | Gas | Gas | Atmospheric air (Oxygen and other gases in Nitrogen) |
| Gas in Liquid | Gas | Liquid | Aerated drinks (Carbon dioxide in water) |
| Liquid in Liquid | Liquid | Liquid | Vinegar (Acetic acid in water), Alcohol in water |
| Solid in Liquid | Solid | Liquid | Tincture of iodine (Iodine in alcohol), Saltwater |
| Solid in Solid | Solid | Solid | Metal alloys like Brass (Zinc in Copper) and Bronze |
Suspensions
A suspension is a heterogeneous mixture in which insoluble solid particles are suspended throughout the bulk of a liquid or gas medium without actually dissolving in it.
Key Characteristics of Suspensions
- Particle Size: The suspended particles are relatively large, with a diameter greater than 1000 nanometers (>1000 nm or >10−6 m).
- Visibility: The particles are large enough to be seen easily with the naked eye.
- Heterogeneity: Non-uniform composition with distinct, visible boundaries between the phases.
- Stability: Unstable; the particles undergo sedimentation and settle down at the bottom due to gravity when left undisturbed.
- Filterability: The large particles can be easily separated from the medium using ordinary filter paper.
Examples of Suspensions
- Chalk powder stirred into water.
- Muddy water or clay particles suspended in water.
- Flour mixed thoroughly in cold water.
- Slaked lime used for whitewashing.
Colloids (Colloidal Solutions)
Colloids occupy an intermediate position between true solutions and suspensions. While they appear to be homogeneous to the naked eye, they are heterogeneous in nature when examined under an ultra-microscope. A colloid consists of a dispersed phase (the suspended particles) and a dispersion medium (the surrounding fluid).
Key Characteristics of Colloids
- Particle Size: The particle size ranges between 1 nanometer and 1000 nanometers (1 nm to 1000 nm).
- Visibility: Individual particles cannot be seen with the naked eye, but their presence can be detected via light scattering.
- Homogeneity/Heterogeneity: Physically looks homogeneous, but microscopically heterogeneous.
- Stability: Quite stable; the dispersed particles do not settle down over time because they carry similar electrical charges that cause them to repel each other.
- Filterability: Particles pass through ordinary filter paper easily but can be separated using specialized ultra-filters or cellophane membranes.
Complete Classification of Colloidal Systems
| Dispersed Phase | Dispersion Medium | Technical Name | Common Examples |
|---|---|---|---|
| Solid | Solid | Solid Sol | Colored gemstones, ruby glass |
| Solid | Liquid | Sol | Paints, ink, cell fluids, starch solution |
| Solid | Gas | Aerosol | Smoke, automobile exhaust, dust storms |
| Liquid | Solid | Gel | Cheese, butter, boot polish, fruit jellies |
| Liquid | Liquid | Emulsion | Milk, face cream, cold cream, cod liver oil |
| Liquid | Gas | Aerosol | Fog, mist, clouds, insecticide sprays |
| Gas | Solid | Solid Foam | Pumice stone, foam rubber, styrofoam |
| Gas | Liquid | Foam | Soap bubbles, whipped cream, shaving cream |
Comparative Summary of Solutions, Colloids, and Suspensions
| Property | True Solution | Colloidal Solution | Suspension |
|---|---|---|---|
| Nature | Homogeneous | Heterogeneous (Appears homogeneous) | Heterogeneous |
| Particle Size | <1 nm | 1 nm to 1000 nm | >1000 nm |
| Sedimentation | Does not settle | Does not settle | Settles on standing |
| Separation by Filtration | Not possible | Not possible with ordinary filter paper | Possible with ordinary filter paper |
| Tyndall Effect | Does not exhibit | Exhibits distinctly | Exhibits (until particles settle) |
| Appearance | Clear and transparent | Translucent | Opaque |
| Molecular Movement | Ordinary diffusion | Brownian motion | Settling down due to gravity |
Key Optical and Kinetic Phenomena in Colloids
The Tyndall Effect
The Tyndall effect is the phenomenon of scattering of a beam of light by colloidal particles, making the path of the light beam visible. True solutions do not show this effect because their particles are too small to scatter light waves. Suspensions show it temporarily until the heavy particles completely settle out.
- Prerequisites: The diameter of the dispersed particles must not be much smaller than the wavelength of the light used, and there must be a large difference in the refractive indices of the dispersed phase and the dispersion medium.
- Real-world Examples: * A beam of sunlight entering a dark, dusty room through a small hole.
- The visible beam of car headlights passing through dense fog or smog.
- The blue appearance of clear sky and the bluish tint of diluted milk.
Brownian Motion
Brownian motion is the continuous, rapid, zig-zag, and random movement of colloidal particles suspended in a liquid or a gas medium. First observed by botanist Robert Brown, it is caused by the constant, unequal bombardment of the colloidal particles by the fast-moving molecules of the dispersion medium.
- Significance: This kinetic energy opposes the downward pull of gravity, explaining why colloidal solutions are highly stable and do not undergo automatic sedimentation.
Purification via Dialysis
Dialysis is a clinical and chemical purification process used to separate colloidal particles from crystalloids (true solute particles) by virtue of their differing diffusion rates through a semi-permeable membrane.
- Application: This principle forms the basis of artificial kidney machines (hemodialysis) used to filter metabolic waste products out of human blood when natural kidneys fail.